I have a circuit that I built to control a radiator fan in a car. The circuit uses an lm34 temperature sensor to provide a voltage signal to 2 comparators set up in a modified window switch circuit. Normally the circuit operates like this:

The temperature rises to the "low" set point as determined by R2 of 180 degrees which turns on LED 2 and provides a ground path for the SCR Q1. The temperature continues to rise until it reaches the "high" set point of 200 degrees as determined by R1. At that point LED1 turns on and provides a gate signal for the SCR through Q2. That triggers the SCR into conduction an ultimately turns on the radiator fan. As the temperature starts to drop LED1 turns of followed by LED2 as the temperature drops past the low point and then the SCR turns off.

Everything works fine on the bench until I switch off the florescent bench light. If the circuit is at the point where the fan has turned on and the temperature has dropped past the high point but above the low point and I shut off my bench light the SCR shuts off. Any have any ideas why? Seems to be picking up noise, how can I eliminate it?